The onion’s cells contain an enzyme called alliinase and chemical compounds called sulfoxides. Normally the alliinase and sulfoxides are kept apart, but when the onion is chopped, they are released and react to form sulfenic acid.
This acid is then acted on by a second type of enzyme called a synthase, which produces a gas called ‘syn-propanethial-S-oxide’. This diffuses through the air and irritates your eyes, prompting your tear glands to produce tears to flush it out.
- Why do your eyes water when your nose is hit?
- Why does crying give me a headache?
- Are we the only animals that cry?
- Is crying good for you?